This invention relates generally to tunable optical filters used in spectroscopic apparatus and Fabry Perot etalons as optical elements in particular.
Etalons achieve the highest resolution of any known optical element. Measurements have been made to thousandths of an wavenumber. Etalons are usually constructed so as to have two plane surfaces which face each other. The surfaces are separated by a spacer so as to be parallel, and are coated with a material so as to be partially reflective. A light beam directed to the etalon will be reflected many times between the coated surfaces with the result that only certain frequencies of light will pass through the etalon.
The etalon has the characteristics of a comb filter in that it passes light in a plurality of evenly spaced passbands. The passbands are also known as orders, which are individually numbered and are evenly separated by the free spectral range of the etalon. The separation is measured in wavenumbers.
For application in spectroscopic and like devices it is necessary to adjust the frequency characteristics of the etalon. It is known that both the center wavenumber of a given order and the spacing between orders are dependent upon the distance between the partially reflective surfaces. A number of methods have been devised to alter the spacing between the parallel plates. These methods include the use of pneumatic cells, piezoelectric devices and mechanical arrangements; all employed to effectively move the parallel plates together or apart.
While the methods described work well under most circumstances, they are subject to inaccuracies and call for delicate structure.
Etalons may also be tuned by a method described by Wolga et al. in U.S. Pat. No. 3,914,055. This method calls for an etalon having a fixed plate separation to be rotated by a motor or linkage with respect to fixed light beam. The effect is to change the optical distance between the plates even though the plate separation does not actually change. Tuning by turning the etalon is limited in speed by the inertia of the etalon.
It is an object of the present invention to provide an adjustable etalon without the aforementioned limitations.